I. Field of the Invention
This invention was made with Government support and the Government has certain rights in the invention. This invention relates to a process for making a molecular form of carbon known as fullerenes. A fullerene is a third form of pure carbon different from graphite and diamond, the only two forms known before 1985. In 1985, Richard Smalley and a team of chemists at Rice University identified the structure of one fullerene that contained 60 carbon atoms. This C.sub.60 molecule has come to be known "buckminsterfullerene." See "Fullerenes," Curl, R. F. and Smalley, R. E., Scientific American, Oct., 1991, pp. 54-63, incorporated herein by reference, and references cited therein.
A fullerene structure is characterized in that each carbon atom is bonded to three other carbon atoms. The carbon atoms so joined curve around to form a molecule with a cage-like structure and aromatic properties. A fullerene molecule with 60 carbon atoms resembles the familiar shape of a soccer ball. A structural diagram representing buckminsterfullerene, C.sub.60, is shown in FIG. 1. Fullerenes may contain even numbers of carbon atoms totalling from 20 to 500 or more. FIG. 2 shows the structure of a C.sub.70 fullerene and FIG. 3 shows the structure of a C.sub.84 fullerene. Fullerenes are not necessarily spherical. They may take the form of long tubular structures with hemispherical caps at each end of the tube. Hyperfullerene structures also exist wherein one structure is contained within a second larger structure. For generally spherical molecular structures, these hyperfullerenes resemble an onion layered structure. Tubular structures within larger structures are also possible. Fullerenes are more fully described in the literature cited above.
II. Description of the Prior Art
The molecular structure for buckminsterfullerene was first identified in 1985, see NATURE, C.sub.60 : "Buckminsterfullerene", Kroto, H. W., Heath, J. R., O'Brien, S. C., Curl, R. F. and Smalley, R. E., Vol. 318, No. 6042, pp. 162-163, Nov. 14, 1985. The process described therein for making fullerenes involves vaporizing the carbon from a rotating solid disk of graphite into a high-density helium flow using a focused pulsed laser.
Another method of making fullernes was describe in THE JOURNAL OF PHYSICAL CHEMISTRY, "Characterization of the Soluble All-Carbon Molecules C.sub.60 and C.sub.70, " AJIE et. al, Vol. 94, No. 24, 1990, pp. 8630-8633. The fullerenes are described as being formed when a carbon rod is evaporated by resistive heating under a partial helium atmosphere. The resistive heating of the carbon rod is said to cause the rod to emit a faint gray-white plume. Soot-like material comprising fullerenes is said to collect on glass shields that surround the carbon rod.
A major drawback to these prior art processes is the extremely low quantity of fullerenes produced. Typical fullerene production rates under the best of circumstances using these processes amount to no more than 100 mg/h. Furthermore, the prior art processes are not easily scaled-up to commercially practical systems. The present invention provides a method for producing fullerenes in much larger quantities than have been possible before. The present invention also provides a method for producing fullerenes which can be scaled up to produce commercial quantities of fullerenes. Other disadvantages of the prior art are also overcome by the present invention which provides a method for producing fullerenes in an electric arc.